Preamplifier
On the 80-meter band, we plan to use a special receiving antenna—a Beverage antenna. It is a very long wire (nearly 200 meters) suspended at a low height above the ground, which gives this antenna its directivity—the ability to receive signals only from a specific direction. Its gain is much lower than that of standard antennas mounted at greater heights. To minimize differences in signal strength between the two antennas, it is necessary to use a preamplifier with the beverage antenna.
I built one version of this antenna, and here are the details:
The first very important step is to match the antenna’s impedance (approx. 450 ohms) to the commonly used impedance. The transformer is wound according to W8JI’s recommendations on Amidon dual-hole cores. SWR measurements confirmed its excellent transformation characteristics. You can find instructions for winding the transformer at http://www.w8ji.com/k9ay_flag_pennant_ewe.htm
Preamp Circuit Diagram for a Beverage Antenna
To prevent the preamplifier from being excited by unwanted out-of-band signals, a 3.6–3.98 MHz bandpass filter is connected to its input, and the entire unit is housed in a shielded box. This filter has excellent performance and is easy to tune. However, calculating such a filter is complicated. You can find the calculation here: http://www.cq.sk/modules.php?name=News&file=article&sid=135
Mechanical Layout Inside the Enclosure
The preamplifier itself is very simple and uses an MMIC integrated circuit. The circuit used has a gain of approximately 8 dB. More gain is not necessary; otherwise, strong signals could overload the receiver’s input. Milan OK1VWK explained this issue very well at: http://www.sweb.cz/ok1vwk/K9AY%20loop%20-%20modifikace%20OK1VWK.htm
To prevent the Beverage antenna from being bidirectional—that is, from receiving signals from two directions—one end must be loaded with a non-inductive resistor. Such a resistor has an impedance of approximately 450 ohms and, due to weather conditions, is housed in a waterproof box.
The figures show the circuit configuration without a 9:1 input transformer and the supply voltage indicators (LED + resistor:
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